In a post earlier this month Timothy Boronczyk took a look at a common feature for web applications that's popping up more and more given the emphasis of social networking - geolocation of your users. The post shows you how to use various web services and data sets to determine where a user is and how they relate to other locations.

Services that allow users to identify nearby points of interest continue to grow in popularity. I'm sure we're all familiar with social websites that let you search for the profiles of people near a postal code, or mobile applications that use geolocation to identify Thai restaurants within walking distance. [...] The first step is to obtain the latitude and longitude coordinates of any locations you want to make searchable.

He talks about why postal code-based location isn't the best option, a web service (like Geocoder.ca for Canadians) could provide more accurate results. He includes code you can use to help search the data based off of two formulas - one to grab the larger area of locations and the other to filter them down into the closest. Finally, he shows how to use the Geolocation API service to get the results based on the user's search parameters.